The vast majority of organic polymeric materials undergo some mode of degradation when exposed to the high energy photons of ultraviolet radiation. This degradation manifests itself, depending upon the polymeric material, in yellowing, embrittlement, and other loss of physical and mechanical properties. Polycarbonate resins are no exception to this degradation by ultraviolet radiation.
While polycarbonate resins exhibit many excellent mechanical properties which render them suitable for use as thermoplastic emgineering materials, glazing materials, and films, their propensity towards degradation by ultraviolet radiation restricts their use in applications where they are exposed to sunlight. This is particularly true in the case where the polycarbonate resins are used as glazing materials or films which are exposed, by necessity, to sunlight.
The use of ultraviolet radiation screeners with various polymeric materials, including polycarbonates, to provide protection against degradation by ultraviolet radiation is known in the art. The ultraviolet radiation absorber or screener functions by reason of its ability to screen out or absorb the damaging ultraviolet portion of light due to its very high absorptivity in this range of the spectrum relative to that of the polymer. In order to qualify as a successful ultraviolet radiation screener for a polymer, particularly for polycarbonate resins, the screener must fulfill several requirements. The screener must have a high specific absorptivity in the range of wavelengths that are most deleterious to the polymer, it must be compatible with the polymer such as polycarbonate and must not degrade or adversely affect the polymer, the screener must not significantly absorb in the visible light region of the spectrum or a color will be imparted to the resin, and the screener must have a sufficiently low volatility so as to permit its continued presence in the polymer during processing of the polymer.
Several methods are known in the art and conventionally utilized for employing these ultraviolet radiation screeners to protect various organic resins against degradation by ultraviolet radiation. These methods include the blending of an ultraviolet radiation screener with the polymer prior to processing of the polymer, incorporating the screeners in surface coatings which are applied onto the surface of the polymer, and impregnating the surface of the polymer with the ultraviolet radiation screeners.
While each of these conventional prior art methods can be utilized to impart improved improved ultraviolet radiation stability to a polymer they all have certain disadvantages. Blending the absorber with the bulk polymer results in the absorber being distributed throughout the entire polymer. This procedure is both uneconomical, as these screeners are generally quite expensive, and not completely successful. Since most of the screener resides in the polymer's interior instead of at the surface of the polymer where it is most needed, much of the harmful ultraviolet radiation penetrates and deteriorates the surface areas of the polymer before reaching and being absorbed by the majority of the interiorly distributed screener. Furthermore, since the concentration of the screener in the resin is limited by the degree of compatibility of the screener with the polymer,using sufficiently high concentrations of screener effective to provide surface protection generally tends to adversely affect some of the advantageous mechanical properties of the polymer.
Incorporating the screeners in surface coatings suffers from the disadvantage of being difficult and expensive to use since an extra and complicated processing step is required. Furthermore, difficulties are sometimes encountered in adhering the coating to the surface of the polymer or in maintaining continued adequate adhesion, especially after prolonged exposure to weathering. Even when the coating material adheres well to the polymer and does not delaminate after weathering it often cannot be applied without forming unsightly streaks on the polymer surface.
While, in theory, the surface impregnation techniques are the most desirable since the ultraviolet radiation screeners are contained only in the surface regions of the polymer where they are most needed, in practice the conventional surface impregnation techniques also suffer from several disadvantages. Melting the polymer and the screeners in order to diffuse the screeners into the polymer surface suffers from the disadvantage that the polymer, or at least its surface layers, must be heated to the melting point. This may result in an uneven or wrinkled surface being formed upon cooling and solidifying of the surface. Furthermore, some of the advantageous mechanical properties of the polymer may be adversely affected by this melting of the processed polymer surface. In the aggressive solvent technique an ultraviolet radiation screener is dissolved in a solvent which is aggressive towards the polymer such as polycarbonate. Typical aggressive solvents for polycarbonates include chlorinated hydrocarbons, esters, or aromatic hydrocarbons. When these solutions are applied onto the surface of a polycarbonate article the aggressive solvent functions as a softening or swelling agent for the polymer surface allowing the screener to diffuse into the softened or swelled surface regions. However, the aggressive nature of these solvents causes problems. Surface imperfections can occur during impregnation and prolonged contact between the polymer and the aggressive solvent can lead to etching, hazing, and environmental stress cracking and crazing of the polymer. While the processes for surface impregnation of ultraviolet radiation screeners into polycarbonate articles described in U.S. Pat. Nos. 4,323,597 and 4,322,455, to Olson et al., which processes include contacting a solution containing an ultraviolet radiation screener and a non-aggressive solvent with a polycarbonate article which has been heated, overcome the disadvantages described hereinafore, they nevertheless require an additional step in the preparation and processing of the polycarbonate article and are time consuming and add to the expense of the finished product.
It is, therefore, an object of the instant invention to provide a method for the surface impregnation of ultraviolet radiation screeners into the surface layers of polycarbonate articles which overcomes the aforedescribed disadvantages, does not require an added processing step, and is economically and easily accomplished.